Large quantities of H2S-containing gases are commonly produced in the natural gas and petroleum industry and concentrated by amine treating units and sour water stripping units. Claus sulfur recovery plants (“Claus plants”) are in widespread use to convert this environmentally hazardous H2S to useful elemental sulfur by oxidation according to the overall or net equationH2S+½O2→1/xSx+H2O  (1)wherein x=2, 6 or 8, depending on the particular conditions of temperature and pressure. The net production of elemental sulfur is usually accomplished as a series of process steps carried out according to a conventional plant flow scheme. A conventional Claus unit comprises a free flame combustion/reaction furnace stage and a catalytic stage.
The free flame combustion step takes place by burning ⅓ of the H2S in burner according to the equation:H2S+ 3/2O2→SO2+H2O  (2).Oxygen for the combustion stage is usually supplied by air from an air compressor or blower. The combustion stage is followed by the stages in which the “Claus reaction” takes place according to the equation2H2S+SO23/xSx+2H2O  (3)wherein x=2, 6 or 8, depending on the particular conditions of temperature and pressure.
The Claus reaction initially takes place in the reaction furnace immediately following the burner, and while the gases are at near-flame temperatures. After the gases exit the reaction furnace they are cooled in a waste heat boiler (WHB), usually with boiling water circulating in the waste heat boiler and being converted to medium to high-pressure steam. After cooling, the gases are cooled further in a sulfur condenser, in which boiling water is circulated to make low pressure steam. At this stage in the process about 50-70% of the incoming H2S will typically have been converted to elemental sulfur. The actual amount depends on such factors as inlet H2S concentration, flame temperature, residence time in the reaction furnace following the burner, and the presence and amount of other chemicals such as other combustibles or carbon dioxide. Condensed liquid sulfur product is usually recovered at this point in the process.
A 70% level of conversion is insufficient by today's standards to allow the effluent from the Claus furnace to be emitted to the atmosphere or to make tail gas treatment economical at this point. An increase in the overall level of conversion is usually achieved by removing one of the reaction products from the mixture (e.g., by condensing and removing liquid elemental sulfur), and then allowing the remaining gases to continue reacting until equilibrium is reached (Equation 3). After the reaction furnace, the reacted gases are cooled in a WHB against boiling water. The gases can be cooled to allow condensation of sulfur in this WHB, or, more typically, the cooled gases from the WHB are further cooled in a separate sulfur condenser to facilitate condensation of the sulfur formed in the first reaction stage.
In modified Claus plants, further recovery of sulfur is accomplished by taking the gases from the first condenser, reheating, and then passing the gases over a high surface area Claus catalyst in a packed bed reactor. The Claus reaction (Equation 3) takes place on the catalyst up to the equilibrium limit of the reaction. Some well-known Claus catalysts are bauxite, alumina and titania. The Claus catalytic reactors are normally operated in the gas phase to prevent condensed sulfur from plugging the pores of the catalyst. To enhance recovery of sulfur via the Claus reaction, the elemental sulfur is conventionally removed by condensation in a sulfur condenser which follows the catalytic reactor. Similar reheat, reaction and condensation steps are commonly repeated two to three times in order to maximize sulfur yield of the plant. Because of the equilibrium restraints inherent in the Claus reaction (Equation 3), adding more catalytic Claus reactors becomes ineffective beyond a total of three or four units, so other measures must be taken in order to further increase sulfur recovery beyond about 98 vol. % of the initial H2S and to complete the recovery of the remaining sulfur before the effluent is released to the atmosphere.
The addition of equipment needed to improve recovery almost invariably decreases the capacity of the plant by adding resistance to flow from additional friction. Thus the addition of each reheater, catalytic Claus reactor, sulfur condenser and tail gas treatment unit is accompanied by a reduction in operating pressure. Moreover, as demand for sulfur recovery capacity grows in an existing facility, the flows of O2-containing gas and H2S-containing gas into the Claus plant will increase. This increase in flow causes an increase in pressure drop through the system approximated by the relationshipDP2/DP1=(Q2/Q1)2  (4)where DP is pressure drop, Q is volumetric flow rate, 1 is the initial flow condition, and 2 is the new flow condition. In any given system, at a certain flow rate of H2S-containing gas the pressure drop due to friction from flow will exceed the available pressure drop through the unit. At that point, the unit is capacity constrained. Conventional Claus plants operate at low pressure, usually 20-30 psia at the front of the plant. In almost every case, a conventional sulfur recovery plant with a burner, reaction furnace, multiple reheat, catalytic Claus reactor, and condenser stages, and single tail gas treatment unit is limited to 5 to 15 psi of available pressure drop. Many existing Claus plants suffer from a severe constraint in capacity.
Following LeChatelier's principle, the flame and reaction furnace section of the furnace should be operated at the highest temperature possible to drive the equilibrium conversion of sulfur. This temperature is usually regulated by the incoming reactant temperatures, by the concentration of H2S and other combustible gases, such as light hydrocarbons, and the presence of inerts in either the H2S-containing gas or in the air. It is assumed in Claus design that as the reaction mixture cools in the waste heat boiler following the reaction furnace, the mixture will be at or near equilibrium and the mixture will retain this composition by the rapid cooling in the waste heat boiler “quenching” the reaction.
Another assumption is that the formation of sulfur in the reaction furnace/waste heat boiler will inhibit the formation of sulfur in subsequent catalytic stages according to LeChatelier's principle; that is, sulfur is a reaction product, so having sulfur in this stream will shift the reaction equilibrium the wrong direction if kept in the process stream. Therefore, the waste heat boiler is normally built with extra heat transfer capability to condense the bulk of the sulfur vapor formed, or a sulfur condenser after the waste heat exchanger is added. It is also typical to reduce the temperature of the gases from the condenser to get the maximum amount of sulfur out of the gas stream before proceeding to the next conversion stage. Simplification of the Claus process by removing pieces of equipment in the apparatus and process flow can be beneficial by reducing the cost of equipment and by decreasing the frictional resistance to flow thereby increasing unit capacity.